Accretion of ice on the surface of aircraft is a problem experienced by flight operations. Such ice accretion may dramatically impact the performance characteristics of the aircraft and has been known as the root cause of a number of major accidents in recent times.
Accretion of ice may occur when the aircraft is flying through supercooled liquid such as cloud, rain or drizzle and at a time when the temperature of the surface of the aircraft is below freezing point. The severity of the ice accretion is dependent on the supercooled water content, the temperature and the size of the cloud droplets or raindrops. Ice accretion on an aircraft surface increases drag, alters aircraft handling characteristics, disrupts the airflow, reduces lift and may result in a stall condition.
Some large Transport and Regional Aircraft are currently equipped with an inflight ice detection system to warn of ice accretion on the surface of the aircraft. Typically such systems are based on magnetostriction principles to drive a sensing probe at an ultrasonic frequency. As the probe accretes ice the probe frequency decreases due to the increased mass. This change in frequency initiates the icing signal. Such systems are expensive, consume considerable power and occupy significant space in the aircraft.
Within Business and General Aviation Aircraft operations the practise of direct pilot observation of ice after it has accumulated on the aircraft wing is commonly found. The pilot incurs a substantial distraction when required to make such observations. Such an observation and related distraction may occur at a critical phase of the flight.
At the present time there are no known, reliable, un-attended and cost-effective inflight ice detection systems available for the complete range of aircraft from Transport Category to General Aviation type aircraft.
This invention introduces an improved inflight ice detection system, of lower cost and simpler construction, capable of being utilised, un-attended, on the complete range of aircraft in operation today. Additionally this invention provides an improved positive discrimination between ice and other substances such as water or de-icing fluid that may be found on the surface of the aircraft whilst inflight.
Recent advances in Inflight Integrated Detection Systems such as that disclosed in U.S. Pat. No. 6,430,996 act to detect, inflight, a number of different parameters including the presence of ice using multi function sensor probe. Such equipment detects ice when a light beam, directed across a recessed surface within the probe, is broken. While such a system provides additional information the function of inflight icing detection does not provide a reliable detection of ice on the surface of aircraft because ice formation on the recessed surface of the probe lags ice formation on prominent aircraft surfaces. Neither does the method provide an effective discrimination between water and ice.
Recent advances, such as those disclosed in European Patent No. EP1633626, International Publication Number WO 2004/110865 A1, act to detect ice on the surface of an aircraft by use of a single emitter connected to the surface and a six piece sensor array, connected to the surface by optical fibres at different distances on the surface with respect to the emitter. A complex means of detecting the distribution of the reflected and scattered light across the six sensors determines the presence, type and thickness of ice on the surface. Such a system requires significant set-up and is prone to changes from calibrated conditions. Additionally such a system does not discriminate between water and clear ice. Such a system is impractical for use inflight on the complete range of aircraft.
Recent advances in Inflight Detection of Icing Conditions such as that disclosed in U.S. Pat. No. 6,091,335 act to detect, inflight, in an external volume from the aircraft, likely icing conditions. Such equipment detects possible icing conditions as indicated by the concentration of water and ice particles in the external illuminated volume. The icing severity is determined by the temperature and the water concentration multiplied by speed of aircraft. Such a system utilises separate transmit and receive optics known in the art as bi-static telescope arrangements. Those experienced in the art favour the use of mono-static telescope arrangement due to difficulties in maintaining critical alignment between transmit and receive optics. While such a system may provide information on the risk of icing conditions in volumes external to the aircraft it does not provide a reliable measure for detection of inflight ice accretion on the surface of an aircraft.
Advances in Inflight Ice Detection Systems such as that disclosed in U.S. Pat. No. 6,052,056 act to detect and warn the pilot of the presence of substances on the surface of the aircraft. Such equipment detects changes from a standard pattern and infers the presence of ice by a variation in the amount of light returned from the monitored surface. While such a system, containing a single light detector is compact it cannot discriminate contamination from ice.
Advances, such as those disclosed in U.S. Pat. No. 6,010,095 act to detect ice by means of total internal reflection of a light beam. As the refractive index of water and ice are very similar this and other advances using refraction techniques does not have the ability to effectively discriminate between ice and water. The use inflight of such equipment for detection of ice on aircraft surfaces is not appropriate.
Advances, such as those disclosed in U.S. Pat. No. 6,069,565 act to detect ice on the metallic painted surface of an aircraft by transmitting light from a strobed source to the surface of the aircraft and then splitting the reflected light into an isolated portion and a non-isolated portion. A complex means of delaying one portion relative to the other and comparing the received signal of the two portions is used to determine the presence of ice. Such equipment may be used by an operator to detect presence of ice on an aircraft surface prior to flight. Unfortunately, such equipment cannot be used in an un-attended fashion inflight.
Other advances, such as those disclosed in U.S. Pat. No. 5,850,284 act to detect ice on the surface of an aircraft by placing a polarization filter on the surface of the aircraft and then detecting the reflections of ambient light on that surface. Significant manipulation by the equipment operator is required in order to detect ice on an aircraft surface prior to flight. Unfortunately, such equipment is unable to detect inflight icing conditions in an un-attended fashion.
Other advances, such as those disclosed in U.S. Pat. No. 5,841,538 act to detect ice on the surface of an aircraft by placing a retroreflector on the aircraft surface and then placing a polarizing filter on the retroreflector. The reflections of light are viewed when passed through a second filter placed in the reflection path. Significant equipment manipulation is required to detect the presence of ice on the surface of the aircraft prior to flight. Unfortunately, such equipment is impractical for detection of inflight icing conditions in an un-attended fashion.
Advances, such as those disclosed in U.S. Pat. No. 4,980,673 and related U.S. Pat. No. 5,003,295 act to detect ice deposition utilising thermal sensing and related thermal control systems. While such equipment may be used inflight it's use is not accurate as in flight ice deposition can occur over a very wide range of temperature and flight conditions.
Advances, such as those disclosed in U.S. Pat. No. 5,243,185 act to detect ice by transmitting linear-polarized light to a target surface, filtering the single channel reflections with an elliptically-polarized filter and then detecting the filtered reflections via a number of sensor elements. Each of the sensor elements detects the reflected response from different spatial locations. The variance among the different sensor signals is processed as an indication of the variation in filtered reflections across the target. Unfortunately, in view of the randomness of ice crystal orientations, a variance will not be detected unless the sensors are set to detect at individual crystal level. Such a system requires many sets of acquisitions, using additional filtering, over the same target area in order to give adequate accuracy of detection. Such a system, requiring significant processing, may be usable to test for presence of ice prior to flight. Such a system however is impracticable for use inflight on the complete range of aircraft.